11C-methyl-L-methionine (C-11 Met)PET, CT and MR imaging were performed in eleven patients with malignant brain tumors before and after RT to evaluate the usefulness of positron emission tomography (PET) in monitoring tumor response to radiotherapy (RT). The subjects included five cases of intracranial malignant lymphoma (ICML) and six cases of glioma. C-11 Met uptake by the tumor (T) and the contralateral gray matter (NT) was calculated on the PET images. The mean T/NT ratio of the ICMLs and gliomas changed from 2.33 and 1.87, respectively, before RT to 1.31 and 1.58, respectively, after RT.No significant difference was found between the T/NT ratios before and after RT in either the ICMLs or the gliomas (t-test). We tentatively defined the minimum T/NT ratio, 1.2, as the threshold between tumor and nontumor regions. Tumors with a ratio of 1.2 or more were imaged as "hot" (MET) and coincided with CT or MR image lesions which were visualized as contrast-enhancing (CE) and low-density (LD) or high-intensity (HI). The relationships between PET, CT and MRI lesions were classified as follows: Type I (MET < or = CE), Type II [CE < MET < LD (HI)], Type III [LD(HI) < or = MET]. MET lesions extending regionally ( > 1 cm) beyond the respective CT or MR image lesions were designated "MET-extension" and LD (HI) lesions protruding ( > 1 cm) beyond the MET lesions were recorded as "LD (HI)-extension" on the integrated images. The type II pattern of the MET areas in all five cases of ICML before RT had changed to Type I in one case, Type III in one case and Type II in three cases, after RT, while the two Type II patterns and four Type III patterns of the gliomas had converted to four Type II and two Type III patterns. These findings indicate that gliomas tend to invade into areas of peritumoral edema more than ICMLs. There were two ICML MET-extension sites in the cortex before RT, as opposed to two in the cortex, one in the basal ganglia, one in the thalamus, and one in the corpus callosum among the gliomas. On follow-up CT or MR images MET-extension (75%) had converted to a CE or LD (HI) region. Four ICML LD (HI)-extension sites before RT were found in periventricular white matter, versus one in the cortex and three in the white matter among the gliomas. LD (HI)-extension appeared to represent vascular edema because it decreased or diminished after completion of therapy. Sequential analysis of integrated C-11 Met PET, CT and MR images is useful in detecting the extent of tumor infiltration by ICMLs and gliomas, particularly at an early stage, and for evaluating the effect of RT in the treatment of both.